In Doppler ultrasound, the term "aliasing" refers to what phenomenon?

In Doppler ultrasound, the term "aliasing" refers specifically to an incorrect speed interpretation that occurs when the sampled frequency exceeds the Nyquist limit. The Nyquist limit is defined as half the pulse repetition frequency (PRF). When blood flow velocities exceed this limit during Doppler imaging, the system can no longer accurately represent the true frequency of the returning echoes. As a result, instead of accurately displaying the higher velocities, the system interprets them as lower velocities in the opposite direction, which leads to the creation of ghost images or wrap-around effects. This phenomenon appears as a reversal or incorrect colors in color Doppler imaging, indicating that the flow is in a direction opposite to what is actually occurring.

Understanding aliasing is crucial for clinicians because it directly impacts the accuracy of blood flow measurements and can lead to misinterpretations of the severity of vascular conditions. Careful adjustment of the system settings, such as increasing the PRF or adjusting the angle of insonation, can help mitigate the effects of aliasing and provide a more accurate representation of high-velocity blood flow.